Tuesday, March 11, 2014

Invasion of the Garden Snatchers: The Perennial Problem of Invasive Plants

The introduction of non-native plants in any ecosystem can be a mixed blessing. While many of the old garden and hybrid roses are an example of a blessing, the same cannot be said for kudzu (Pueraria lobata), for example, whose growth rate is best described as meteoric.

Originally introduced into this country by the Soil Erosion Service and Civilian Conservation Corp in 1876 to help control soil erosion problems in Pennsylvania, its rapid growth and luscious foliage made it a favorite to cover shade porches and pergolas in the southern states. It soon became a favorite cover crop because of its rapid growth rate but when left to its own devices and allowed to grow unchecked, it became a veritable menace, destroying everything in its path.

Kudzu has overtaken more than 7.5 million acres in the southern states where it has smothered all low growing plants in its path and covered, broke branches from, and even uprooted trees. Growing up to a foot a day, it doesn't take long to cover an abandoned barn and overtake and damage electrical and phone lines.

Tall reeds mid left of the photograph, 2009, Ipswich River

Kudzu has been identified as far north as New York City but fortunately, it can't survive New England winters. Still, we have our own predatory weeds that more than make up for it.

Here in Massachusetts and along the coast extending into southern New Hampshire, a major and pressing problem is the common reed phragmites (Phragmites australis) which has been rapidly and relentlessly overtaking both salt marshes and freshwater tidal basins alike. Its rapid proliferation has caused serious problems along the entire Atlantic coast but it is of particular concern to us as it now poses a major threat to the survival of the 18,000 acre Great Marsh here in Essex County, a critical part of our local ecosystem and marine culture.

There is a subspecies of phragmites that is native to North America, however the subspecies (P. australis subs. Americanus) is a much less vigorous and more easily controlled plant than Phragmites australis subs. Australis, the non-native variety that is now considered a serious environmental threat.

Phragmites have changed the landscape along the river's edge.

Phragmites reeds grow in dense swaths that can spread as much as 16 linear feet in a year. Mature plants of the non-native variety can reach 15-20 feet in height and are easily identified because except for trees, they are the tallest things you'll find. The subspecies Americanus is much shorter, growing to a maximum height of 6-12 feet.

Phragmites spread and multiply both by seeds (less so) and thick rhizomes (most commonly and aggressively) that are thick, notoriously difficult to kill, and can extend up to 20 inches in depth where they send out innumerable runners. Chopping back the top of the plant does not affect the rhizome which will continue to extend runners and then send up new, denser, more vigorous growth the next growing season. Most importantly, phragmites choke out the other native species which are a critical part of the food chain for local fish and fowl and there is concern about the survival of wildlife and the Marsh if the proliferation continues unchecked.

Normal plant growth is gradually being overtaken by phragmites along the rivers.

Phragmites have been used as a grazing crop (albeit nutritionally incomplete) for livestock, but grazing (and mowing) only increases the vigor of the plants, so cutting down the massive stands only makes the problem worse. And livestock don't graze in the Great Marsh. Extensive burning over the course of multiple seasons appears to be the best chance of achieving control of a plant that tolerates sand, mud, and clay, is highly salt-tolerant, and can also thrive in a wide range of pH.

I first noticed phragmites in 2009 while canoeing on the Ipswich River. (They were certainly here long before that, but had escaped my awareness until then.) I noticed the changing landscape along the river's edge -- the wildflowers and indigenous plants and shrubs were being replaced by a tall grass that formed a dense wall that obstructed the beautiful landscape.

Over the course of two years, large swaths of this noxious weed replaced the natural river habitat in many areas and have completely obliterated the view of the coastal marshes (and everything that used to grow there) along local route US-1.

Even more frustrating is that it found it's way into our own garden this past summer. I went out to weed one of the perennial beds and to my horror,several tall fronds had sprouted in the midst of our Montauk daisies and coneflowers. Digging them out was a major chore and we could not have done it without our spearhead spade shovel.

We abut a meadow that is marshy in the spring and I have seen phragmites growing there, although they are usually mowed down during the twice yearly mowing for hay. Digging them out was a tremendous chore and we ended up digging up and sacrificing several daisies and coneflowers in the process.

Loosestrife sprouted in a rose bed.

Purple Loosestrife (Lythrum salicaria) is another lovely but aggressively invasive plant that has been an increasing and problematic presence over the past several decades. It was introduced into the east coast of the US in the early 1800's and since then, it has spread along waterways and roadways to virtually everywhere in the continental United States save Florida.

A common sight along highway medians, roadsides, waterways, ponds, swamps, and meadows, it's pretty lavender color brightens the roadside. But like phragmites, it overtakes wetlands and marshes and has become a major threat to the ecosystem. Also like phragmites, the root system is exceedingly vigorous and once the plants are established, it can be difficult to dig out even with a spade shovel.

Loosestrife spreads primarily by seed which it produdes in voluminous amounts. Easily carried on the wind or by birds, volunteers have been finding their way into our yard from the adjacent meadow for several years.

Loosestrife popped up unexpectedly in a shade garden

If you find it blooming in your garden, cut the plant back before it blooms and then dig as much of the root system as you can find. If you miss a substantial part of the root, it will re-grow the following season, so keep an eye out early in the growing season and if you see it sprouting, dig early and dig wide.

Loosestrife has popped up in our shaded cottage bed and among our blueberry shrubs. It also has appeared in our sunny rose beds and along the edge of our property where it abuts the meadow. Since there are large clumps of it in the meadow that will continue to shed seed, all we can do is be vigilant and dig it up as soon as we notice it.

In the case of loosestrife, mowing does help, and when we see it sprouting near the property line, we try to keep it mowed to prevent it from blooming and setting seed.

Brilliant fall foliage

The ubiquitous burning bush, Euonymus alatus, can be found dotting the landscape throughout New England.

For decades, its brilliant red autumn foliage and hardy growth habit made it a favorite of landscapers who were designing low maintenance plantings for shopping malls, office buildings, and other places where pollution and drought tolerant shrubs would thrive. The stunning scarlet fall foliage became a favorite of home gardeners as well.

Unfortunately, the burning bush has become a bane of green space and woods alike and many states, Massachusetts included, now restrict its importation, propagation, and sale.

The tendency toward being invasive is primarily a problem where the shrubs have been allowed to naturalize along highways or in pastures and woodlands where they out-compete and eventually replace native plants. When they are planted in urban areas as ornamentals, there doesn't seem to be as much of an issue with them growing out of control, although birds have been credited with spreading the seeds contained in the fall berries.

Three of our four burning bush shrubs showing vivd green foliage in spring.

There were four mature burning bushes already planted in front of our suburban home when Steve originally purchased the property and in the twelve years that we've resided here, we've not had a single seedling develop from any of them.

Many of the problems with the burning bush developed when homes were abandoned and previously tended gardens were left to their own devices. Burning bush berries spread the seed into the woods, likely with the help of birds, and when they sprouted, the rapidly growing plants thrived and began to out-compete the native shrubs for space.

I've long been very concerned about well-intended but sometimes misguided planting of non-native plants in areas where they will be left to their own devices and not cultivated or kept in line. They're kind of like teenagers... you need to set firm limits and if you don't, you end up with a teen (or a burning bush) that is totally out of control. Not a good thing.

We've been asked many times why we, as responsible home owners and
stewards of the land, don't simply dig them up. My feelings about doing
so are complicated. The reality is that any robust plant, native or not, can become invasive if not properly monitored and controlled. We saw this first hand when a tall and vigorous cultivar of Mondara (bee balm) overtook one of our cottage beds. It took six years for us to completely eradicate it from our property and reclaim the bed.

While I doubt that I would ever plant another burning bush (or any more bee balm) even in a locale that allowed them (I've never been particularly fond of burning bushes, their status as invasive aside), I see no value to disrupting the existing garden beds and destroying healthy plants that have to date not caused a problem in their present location, especially since they have historically provided nesting places for birds.

Moreover, at least half of the homes and two shopping malls within three quarters of a mile of our home are prominently planted with them. Destroying these four shrubs might make a statement, but to what outcome?

A large burning bush growing in the cottage garden (2009)

Were the Commonwealth to mandate that all shrubs currently growing in our locale be destroyed, we would comply, but that is not the current recommendation. It seems to us that a more appropriate tact for us to take is to continue what we have been doing for the past several years and that is to exercise a firm hand over those plants that were planted as ornamentals when it was legal and permissible to do so.

What we have found very effective and at the same time uniquely attractive is to drastically prune the shrubs in spring, just as the rather inconspicuous
blooms are opening. This prevents all but a rare few berries from
developing and virtually eliminates the primary way that the burning bush self propagates.

Exposing the inner, larger branches gives the shrub a completely new look and most people who see the shrubs after pruning don't
recognize them.

The same shrub (2012) after we began drastic pruning. Opening up the lower 2/3 of the shrub lightens the visual weight of it in the garden and in so doing, we remove all of the blossoms that eventually would develop into berries and seed.

We leave a generous canopy but shape the top. This shrub was formally thickly branched and as wide as the canopy all hte way down to its base.

4 comments:

This is such an interesting post as the purple loosestrife and the euonymous are plants you purchase from the nursery over here. I had an euonymous alatus in my previous garden and sadly it didn't seed once The native euonymous europeas, or common spindle, grows in the hedgerows here and it's beautiful seedpods are much appreciated. For us the major problem is Japanese knotweed; insurance companies will not insure your house if there is knotweed in the garden. The local councils attack it with heavy duty chemicals if it is found in the countryside. Another problem is the Oxford ragwort (spread along the railway lines during Victorian times) which is poisonous and kills horses.

It can be a challenge to even know what is "invasive" and what is "safe". The list of invasives is constantly changing and varies from one geographic area nad climate to another. As Helen (above) noted, loosestrife is not an issue where she lives (in the UK) and it's available at the garden center there. Here in MA, there were some things on the list in the past (like creeping Jenny) that are not on the list any longer.

Monarda is not on any list that I'm aware of. Yet, we had a major problem here with bee balm that took us 6 years to completely eradicate but not before it very nearly destroyed a cottage garden and a wonderful collection of iris. ;(

But like you, for new plantings, I do avoid known invasives and even things that are not identified as such but that I find I have a tough time controlling in my specific area.

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